Intrusion alarm systems of many different types are known for providing protection of an area against unauthorized entrance. Many prior art systems have the capability to monitor several different protected areas simultaneously to provide an alarm if an intrusion is detected in any of the areas.
Intrusion alarm systems typically detect the presence of an intruder by transmitting an electromagnetic or an accoustic signal into an area and receiving echoes of the transmitted signal. These echoes are then processed to detect the presence of doppler shifted frequency components which indicate the presence of a moving object in the protected area. Such processing generally includes circuitry for discriminating against common false alarm conditions which produce echoes similar to those produced by a moving intruder, which may include ultrasonic signals falling in the doppler signal frequency range, doppler shifted signals produced by air currents in the protected area, or objects such as blinds, moving rapidly back and forth. Examples of prior art processors are shown in U.S. Pat. Nos. 3,942,178, 4,035,798 and U.S. patent application Ser. No. 947,039 filed Sept. 29, 1978 and assigned to the same assignee as the present application. Due to the complexity of the circuitry necessary to perform such processing, most intrusion detection systems which provide protection of several different areas have only one signal processor, and the signals representative of the echoes received by each of the remote receivers are combined and transmitted along a common cable to the signal processor. While this approach results in an economical system, the processor is unable to determine which receiver has detected an intruder. One example of such a system is shown in U.S. Pat. No. 3,736,584.
In an attempt to solve this problem, intrusion detection systems have been developed which include signal processing circuitry at the receivers to provide an indication of which processor has detected an intrusion. Such systems are exemplified by U.S. Pat. Nos. 3,986,182 and 3,680,074. However, known prior art systems of this type have the disadvantage of requiring complex circuitry and/or requiring large numbers of conductors to be routed around the protected premises.